The present invention relates to the technical field of weaving, and more particularly it relates to the field of fabric intended for industrial use.
Although such a technical field appears to be the most preferable, it should be understood that the present invention can be implemented in other fields, and does not exclude furnishing fabrics or clothing fabrics, for example.
In the preferred field, it is known that the disposition of fibers in a fabric has great influence on the performance of the fabric both in terms of strength in one or more preferred directions, and of suitability for fitting closely over surface shape, particularly when such fabrics are used as reinforcement when making a composite material in association with a matrix, e.g. of resin.
Such abilities are recognized and sought out in order to obtain structural engineering parts of more or less complicated shape by means other than conventional machining, embossing, or stamping which are difficult, lengthy, and expensive to perform, and sometimes also difficult to carry out when the material that is to be used, although of suitable strength, constitutes an insurmountable obstacle because of its mass.
At present, the technique of manufacturing composite material parts is particularly suitable for making engineering parts of more or less complex shape that need to present both light weight and good strength.
Merely as an indication, this applies to hulls, floats, rocket fairings, shrouds for drive units, etc.
Once an attempt is made to comply with the constraints required by such a manufacturing technique, it is clear that optimizing structural characteristics necessarily involves ensuring that the reinforcement constituted by the fabric extends in the proper direction(s) so that the fibers can take up the stresses which will subsequently be imposed on the resulting part in or along the appropriate direction.
Similarly, it is necessary to be able to satisfy the requirement for fitting closely to the shape of the part that is to be obtained, which shape is generally represented by a positive or negative mold on or in which the composite material is to be placed so as to reproduce accurately the shape of the pattern.
This ability to fit closely to the shape of the pattern is generally referred to in the art as xe2x80x9cdrapabilityxe2x80x9d or xe2x80x9clayabilityxe2x80x9d, which is a quality that an engineering fabric must be capable of presenting in association with the preceding requirements.
All of these reasons mean that as a general rule attempts are made to use a fabric whose individual yarns are not interlaced or crossed in conventional manner whereby some of the yarns, generally referred to as xe2x80x9cwarpxe2x80x9d yarns, extend lengthwise relative to the woven cloth while others, generally referred to as xe2x80x9cweftxe2x80x9d yarns, extend crosswise, i.e. across the width of said cloth.
In practice it is often desirable to be able to have fabrics in which individual yarns extend on the bias relative to the above two fundamental directions, with such bias, although generally xc2x145xc2x0 bias, naturally being capable of being subjected to angular variations that facilitate angular spreading of one of the categories of yarns.
In order to satisfy such a requirement using traditional textile manufacturing techniques, proposals have been made to cut individual strips of fabric at an oblique orientation, e.g. at an angle of 45xc2x0, out from a piece of cloth woven in conventional manner. Each strip can then be referred to as a xe2x80x9cbiasxe2x80x9d fabric suitable for being oriented in such a manner that some of its component yarns are placed parallel to certain stresses to which the final part will be subjected.
Such a method is penalizing in several ways.
Firstly, it is clear that the individual strips of bias fabric produced in this way are of finite length, at best equal to the diagonal of a square corresponding to the width of the woven cloth when the orientation used is xc2x145xc2x0. This gives rise to a large fraction of scrap which is very penalizing on the cost price of bias fabric, in particular if the fiber constituting the yarns is expensive to produce.
Another drawback comes from the fact that it is then necessary to place such cutout strips of bias fabric side by side in order to cover a large area, and more particularly a long length. If a uniform structure is to be obtained, the question which then arises is clearly that of making connections between adjacent edges, where such connections must be made in such a manner as to present the same laying, draping characteristics, and also the same strength as that of the fabric. It must be understood that the proposals which have been made for this purpose do not enable the requirement to be satisfied when the composite material for use as reinforcement can or must be formed as a single layer of engineering fabric embedded in a resin matrix. The only way in which adjacent strips can then be connected is to superpose, at least locally, a plurality of layers of bias fabric to form a kind of sandwich at the adjacent edges, thereby significantly increasing the cost price of the final part and giving it dimensional characteristics and strength characteristics that are locally heterogeneous.
Various other propositions have been made in the prior art in attempts to answer the problem as posed above.
Mention can be made of the method of obtaining bias from a unidirectional fabric made up of warp yarns that are small in number relative to the weft yarns.
Such a technique consists in causing the traditional woven cloth to pass between presser rollers beyond which the cloth is taken up by a take-up roller whose axis is inclined at a given angle relative to the direction of the presser rollers.
In order to ensure that the expected results can be obtained durably, it will be understood that it is necessary to make use of fixing or bonding means in the portion lying between the presser rollers and the take-up roller, e.g. means acting where the yarns cross in order to fix the yarns in the newly-imparted orientation. Whatever the means used, such a requirement leads to the bias fabric having significant stiffness so that it is no longer capable of satisfying the requirement of being easy to drape or to lay.
Furthermore, the method used for fixing generally implies adding a bonding material whose presence can have a harmful effect on subsequent behavior of the fabric where it bonds with the matrix of the composite.
Finally, biasing such a conventional fabric necessarily reduces the width of the piece of cloth as initially produced.
Furthermore, such a method gives rise to a fabric in which only one yarn direction is on the bias, which is not exactly the intended object.
Another proposal in the prior art consists in making a fabric with conventional weft yarns and warp yarns but which is produced in the form of a continuous tubular sheath. The principle then consists in cutting such a sheath along a helical path, e.g. oriented at 45xc2x0, so that a woven sheath is obtained which, once opened and laid out flat, has its yarns oriented on the bias. Such a sheath can be produced using a conventional loom or a circular loom.
That technique can be considered as constituting an advance over the preceding technique, but it nevertheless suffers from certain drawbacks.
These include the reworking operation constituted by cutting out helically, which operation must be performed accurately in order to obtain rectilinear selvages. For this purpose, it is necessary to implement means for holding and cutting out both the tubular sheath and the separated portions thereof, which represent a non-negligible industrial cost.
Another drawback relates to the feed means used for feeding the weft yarns, both with a conventional loom and with a circular loom.
As a general rule, such yarns are fed either from spools or previously-prepared bobbins possessing limited winding capacity which means that they must be changed periodically, with the need to be able to reestablish yarn continuity by butt-joining, knotting, bonding, or other technical means. Such operations are penalizing on production, increase cost price, and require the presence, in the resulting cloth, of means for bonding or knotting yarns, which cannot be considered as providing the final fabric with characteristics that are uniform in terms of thickness, flexibility, and strength.
Unfortunately, the need to replace spools or bobbins occurs relatively frequently, given that they are of a capacity that generally makes it possible to produce only relatively short lengths of cloth on a continuous basis without bonding yarn.
Furthermore, using a conventional loom gives rise to an additional problem which is that which stems from unwinding yarn from a bobbin that does not rotate about its own axis. This inevitably gives rise to the yarn being twisted. Such a technique is thus completely unsuitable when the fabric needs to be made from flat yarns, whether such yarns are single-strand or multifilament. Such a weaving method using flat yarns subjected to twisting destroys not only the uniform nature of the resulting fabric in terms of thickness, but also in terms of strength and in ability to be draped or laid.
Proposals have also been in the prior art to make use of a similar method to obtain bias fabric. It consists in using an open braiding machine which does indeed make it possible to obtain a strip in which the interlaced yarns are oriented on the bias relative to the direction in which the strip is produced.
Conventionally, an open braiding machine has yarn feed provided from spools which rotate on their own axes, such that in this case also the same problems arise concerning changing yarn.
Furthermore, in general and as a practical manner, an open braiding machine is incapable of providing cloth of a width that is sufficiently broad to provide a positive solution to requirements for bias fabric, in particular in engineering applications.
Thus, there is still a need to be able to obtain bias fabric that can be produced continuously while avoiding all of the defects and drawbacks that arise using the technical solutions that are presently known.
A specific object of the invention is to produce such fabric in the form of a sheet of uniform thickness, appearance, and technical characteristics so as to be suitable for use in various fields of application that require good ability to be draped or laid so as to fit over a reference surface or pattern.
Another object of the invention is to provide means for obtaining a bias fabric that can be obtained at a production rate that is relatively high, and at a manufacturing cost that is advantageous, and that can be made equally well using single-strand or single-filament yarns or multifilament yarns of regular, uniform, and/or heterogeneous shape, and even from optionally single-strand yarns or roving that is naturally flat or that has been shaped to become flat, which characteristic needs to be preserved in the structure of the resulting bias fabric.
To achieve the above objects, the bias fabric of the invention is characterized in that it is in the form of a cloth of length (L) and of finite width (l), being constituted by interlaced yarns extending in respective directions that are oblique relative to the length (L), and each of which presents no knotting, even for a cloth of indefinite length (L).
The invention also provides a method of continuously manufacturing a bias fabric of above type, such a method being characterized in that it consists in:
warping a sheet of yarns parallel to a direction (x-xxe2x80x2) by causing them to be taken in charge by first and second transfer means occupying mutually parallel directions (y-yxe2x80x2) at an angle (xcex1) with the direction (x-xxe2x80x2);
progressively building up said sheet by placing the yarns successively along a sheet set-up edge;
moving said yarns by the first and second transfer means through one step in the oblique transverse direction from the set-up edge towards an opposite xe2x80x9ctake-upxe2x80x9d edge;
causing the yarn occupying the take-up edge additionally to be taken in charge by a third transfer means situated at a distance from and parallel to the first transfer means in order to act substantially on the middle portion of said yarn;
opening the sheet to form two half-sheets so as to define a sheet close to the first and the third transfer means;
cutting the set-up yarn brought substantially over the take-up edge substantially in the middle portion thereof;
leaving in place that portion of said yarn that is held by the inlet of the third transfer means and by the outlet of the first transfer means to constitute a yarn of the future cloth;
taking the segment of said yarn that is situated between the third and the second transfer means and inserting it into the shed along a direction perpendicular to the direction (x-xxe2x80x2) from the take-up edge towards the set-up edge so as to constitute a yarn of the future cloth; and
proceeding in the same manner in succession with each yarn brought to the take-up edge while also placing a new yarn along the set-up edge and progressively building up an interlace of yarn segments taken in charge by the first and third transfer means and progressing along the direction (xcex1) that is oblique relative to the direction (x-xxe2x80x2).
Finally, the invention also provides a loom for producing a fabric of the above type, the loom being characterized in that it comprises:
a section for warping a sheet of yarns that are set up in succession parallel to one another from a sheet set-up edge;
first and second handling and transverse transfer means extending obliquely relative to said yarns towards a xe2x80x9ctake-upxe2x80x9d edge of the sheet, said means extending parallel to each other along a direction (xcex1) that is oblique relative to the direction of the yarns, said oblique direction defining the direction in which the cloth is produced;
third means set up at a distance from and parallel to the second means and driven in the same direction as the first two means so as to take in charge the substantially middle portion of the yarn of the sheet that occupies the take-up edge;
a section for opening and closing the sheet to form two half-sheets so as to define a shed upstream from the first and second handling means in the cloth production direction;
means serving firstly to cut the yarn of the sheet that occupies the take-up edge at a point upstream from the third handling means, and secondly to leave a portion of yarn for constituting one of the yarns of the cloth in place between the first and third handling means, while also taking hold of the segment of yarn extending between the cut and the second handling means and inserting it into the open shed; and
means for beating each inserted yarn segment and causing it to be taken in charge by the first and third means which guide the resulting bias fabric.
Various other characteristics can be seen in the following description given with reference to the accompanying drawings which show embodiments of the invention as non-limiting examples.